The alternators which are present in all cars have a regulation loop which makes it possible to maintain the voltage of the on-board electrical network at approximately 14 V.
However, the regulators of alternators sometimes include so-called protection functions, such as the detection of excess voltages of the on-board network (>18 V).
Certain regulators cut the generation of current of the alternator, whereas others alert the driver by lighting a red warning light on the dashboard so that the vehicle and the engine can be stopped as quickly as possible.
When an excess voltage is detected by the regulator of the alternator, the excitation must be cut in order to protect the equipment against the excess voltage which could damage both the battery and the connected components.
In most of the present regulators, this function is carried out by processing the signal emitted from the analogue digital converter (CAN). In the oldest regulators, which are known as “analogue”, this function is carried out on the small signals of the regulation loop, which is virtually the equivalent of the aforementioned digital solutions.
However the entire acquisition chain and the digital processing means are on the path of this detection: reference is then made to a common mode for these functions.
In the event of a problem or defect in this common mode, the regulator might not detect this excess voltage, and could contribute to an excess load of the battery, which could damage it, as well as the equipment connected to its terminals.
FIG. 1 shows a general diagram of these known regulators of the prior art.
For example, if resistors R1, R2 or R3 of the detector are shunted and their value is increased to the extent of generating excess voltages, if a capacitor C1 of the filter is short-circuited, the analogue-digital converter CAN supplies erroneous data lower than the expected value, and even if the clock H is blocked with an “ON” command, the architecture can not detect its own faults, since they are in the so-called common mode. All the functions of the regulator, including the power MOSFET which switches the excitation current of the alternator, are potentially affected by this problem associated with the common mode.
To conclude, in the prior art, detection of excess voltage is designed on the principle that the regulator is not defective.
However, the voltage of the on-board network also depends on the elements of the regulator which can have defects.